Internal combustion engines are any of a group of devices in which the reactants of combustion, e.g., oxidizer and fuel, and the products of combustion serve as the working fluids of the engine. The basic components of an internal combustion engine are well known in the art and include the engine block, cylinder head, cylinders, pistons, valves, crankshaft and camshaft. The cylinder heads, cylinders and tops of the pistons typically form combustion chambers into which fuel and oxidizer (e.g., air) is introduced and combustion takes place. Such an engine gains its energy from the heat released during the combustion of the non-reacted working fluids, e.g., the oxidizer-fuel mixture. This process occurs within the engine and is part of the thermodynamic cycle of the device. In all internal combustion engines, useful work is generated from the hot, gaseous products of combustion acting directly on moving surfaces of the engine, such as the top or crown of a piston. Generally, reciprocating motion of the pistons is transferred to rotary motion of a crankshaft via connecting rods.
Internal combustion (IC) engines can be categorized into spark ignition (SI) and compression ignition (CI) engines. SI engines, i.e. typical gasoline engines, use a spark to ignite the air/fuel mixture, while the heat of compression ignites the air/fuel mixture in CI engines, i.e., typically diesel engines.
The most common internal-combustion engine is the four-stroke cycle engine, a conception whose basic design has not changed for more than 100 years old. This is because of its simplicity and outstanding performance as a prime mover in the ground transportation and other industries. In a four-stroke cycle engine, power is recovered from the combustion process in four separate piston movements (strokes) of a single piston. Accordingly, a four stroke cycle engine is defined herein to be an engine which requires four complete strokes of one of more pistons for every expansion (or power) stroke, i.e. for every stroke that delivers power to a crankshaft.
Referring to FIGS. 1-4, an exemplary embodiment of a prior art conventional four stroke cycle internal combustion engine is shown at 10. The engine 10 includes an engine block 12 having the cylinder 14 extending therethrough. The cylinder 14 is sized to receive the reciprocating piston 16 therein. Attached to the top of the cylinder 14 is the cylinder head 18, which includes an inlet valve 20 and an outlet valve 22. The bottom of the cylinder head 18, cylinder 14 and top (or crown 24) of the piston 16 form a combustion chamber 26. On the inlet stroke (FIG. 1), a air/fuel mixture is introduced into the combustion chamber 26 through an intake passage 28 and the inlet valve 20, wherein the mixture is ignited via spark plug 30. The products of combustion are later exhausted through outlet valve 22 and outlet passage 32 on the exhaust stroke (FIG. 4). A connecting rod 34 is pivotally attached at its top distal end 36 to the piston 16. A crankshaft 38 includes a mechanical offset portion called the crankshaft throw 40, which is pivotally attached to the bottom distal end 42 of connecting rod 34. The mechanical linkage of the connecting rod 34 to the piston 16 and crankshaft throw 40 serves to convert the reciprocating motion (as indicated by arrow 44) of the piston 16 to the rotary motion (as indicated by arrow 46) of the crankshaft 38. The crankshaft 38 is mechanically linked (not shown) to an inlet camshaft 48 and an outlet camshaft 50, which precisely control the opening and closing of the inlet valve 20 and outlet valve 22 respectively. The cylinder 14 has a centerline (piston-cylinder axis) 52, which is also the centerline of reciprocation of the piston 16. The crankshaft 38 has a center of rotation (crankshaft axis) 54.
Referring to FIG. 1, with the inlet valve 20 open, the piston 16 first descends (as indicated by the direction of arrow 44) on the intake stroke. A predetermined mass of a flammable mixture of fuel (e.g., gasoline vapor) and air is drawn into the combustion chamber 26 by the partial vacuum thus created. The piston continues to descend until it reaches its bottom dead center (BDC), i.e., the point at which the piston is farthest from the cylinder head 18.
Referring to FIG. 2, with both the inlet 20 and outlet 22 valves closed, the mixture is compressed as the piston 16 ascends (as indicated by the direction of arrow 44) on the compression stroke. As the end of the stroke approaches top dead center (TDC), i.e., the point at which the piston 16 is closest to the cylinder head 18, the volume of the mixture is compressed in this embodiment to one eighth of its initial volume (due to an 8 to 1 Compression Ratio). As the piston approaches TDC, an electric spark is generated across the spark plug (30) gap, which initiates combustion.
Referring to FIG. 3, the power stroke follows with both valves 20 and 22 still closed. The piston 16 is driven downward (as indicated by arrow 44) toward bottom dead center (BDC), due to the expansion of the burning gasses pressing on the crown 24 of the piston 16. The beginning of combustion in conventional engine 10 generally occurs slightly before piston 16 reaches TDC in order to enhance efficiency. When piston 16 reaches TDC, there is a significant clearance volume 60 between the bottom of the cylinder head 18 and the crown 24 of the piston 16.
Referring to FIG. 4, during the exhaust stroke, the ascending piston 16 forces the spent products of combustion through the open outlet (or exhaust) valve 22. The cycle then repeats itself. For this prior art four stroke cycle engine 10, four strokes of each piston 16, i.e. inlet, compression, expansion and exhaust, and two revolutions of the crankshaft 38 are required to complete a cycle, i.e. to provide one power stroke.
Problematically, the overall thermodynamic efficiency of the typical four stroke engine 10 is only about one third (⅓). That is, roughly ⅓ of the fuel energy is delivered to the crankshaft as useful work, ⅓ is lost in waste heat, and ⅓ is lost out of the exhaust. Moreover, with stringent requirements on emissions and the market and legislated need for increased efficiency, engine manufacturers may consider lean-burn technology as a path to increased efficiency. However, as lean-burn is not compatible with the three-way catalyst, the increased NOx emissions from such an approach must be dealt with in some other way.
Referring to FIG. 5, an alternative to the above described conventional four stroke engine is a split-cycle four stroke engine. The split-cycle engine is disclosed generally in U.S. Pat. No. 6,543,225 to Scuderi, titled Split Four Stroke Internal Combustion Engine, filed on Jul. 20, 2001, which is herein incorporated by reference in its entirety.
An exemplary embodiment of the split-cycle engine concept is shown generally at 70. The split-cycle engine 70 replaces two adjacent cylinders of a conventional four-stroke engine with a combination of one compression cylinder 72 and one expansion cylinder 74. These two cylinders 72, 74 would perform their respective functions once per crankshaft 76 revolution. The intake charge would be drawn into the compression cylinder 72 through typical poppet-style valves 78. The compression cylinder piston 73 would pressurize the charge and drive the charge through the crossover passage 80, which acts as the intake port for the expansion cylinder 74. A check valve 82 at the inlet would be used to prevent reverse flow from the crossover passage 80. Valve(s) 84 at the outlet of the crossover passage 80 would control the flow of the pressurized intake charge into the expansion cylinder 74. Spark plug 86 would be ignited soon after the intake charge enters the expansion cylinder 74, and the resulting combustion would drive the expansion cylinder piston 75 down. Exhaust gases would be pumped out of the expansion cylinder through poppet valves 88.
With the split-cycle engine concept, the geometric engine parameters (i.e., bore, stroke, connecting rod length, Compression Ratio, etc.) of the compression and expansion cylinders are generally independent from one another. For example, the crank throws 90, 92 for each cylinder may have different radii and be phased apart from one another with top dead center (TDC) of the expansion cylinder piston 75 occurring prior to TDC of the compression cylinder piston 73. This independence enables the split-cycle engine to potentially achieve higher efficiency levels than the more typical four stroke engines previously described herein.
However, there are many geometric parameters and combinations of parameters in the split-cycle engine. Therefore, further optimization of these parameters is necessary to maximize the performance of the engine.
Accordingly, there is a need for an improved four stroke internal combustion engine, which can enhance efficiency and reduce NOx emission levels.